Antenna radiation plots - What do they mean?

Antennas are a fundamental component of any wireless RF solution. Both the most complex and the simplest wireless RF applications rely on antennas for end to end communication. The quality and performance of the antenna are critical to the correct operation of any wireless system, and it pays to specify it wisely. Selection is often left to the last minute, but design integration is key to getting the best deployment performance.

When designing your latest telematics tracking device or a sophisticated Internet of Things (IoT) end node, the antenna design becomes a critical performance element. Serious consideration needs to be given as to whether to begin a custom design or to use a purchased off the shelf solution. Whichever approach is taken you will inevitably require a radiation plot for evaluation. Below is shown a typical example but what do these plots tell us and what do we need to know?

When manufacturing wireless devices for use globally you will need to undertake therelevant form of certification UL, FCC, CE, etc. and to prove you are not interferingwith other bands and users. In effect, you need to prove the efficiency of your design.Antenna’s are tuned electromagnetic field devices and can be used for single ormultiple frequencies. Due to the nature of modern product design, they take variousforms and shapes. Environmental considerations are critical as performance can andwill be affected by external events such as weather and interference from otherentities.

When producing radiation plots they will come in two basic forms Power Pattern and Field Pattern.

Modern Antenna Design is generally achieved using software such as‘Antenna Magus’ which simulates the elements of the design. The software willproduce a graphical representation of the expected power and field plots. However,external objects will distort these plots and for the final certification, testing will berequired in an Anechoic chamber designed to absorb the reflections ofelectromagnetic radiation, reducing the interference from external spurious sourcesgiving data to support gain and radiation pattern. This article is not intended to coverthe design of the antenna but to interpret the resultant propagation diagrams.

Antenna Fields

The Antenna has different field effects, these are influenced by how close an objectis and the material of the object. The antenna itself will have different radiationpatterns according to frequency, power, and distance. These regions or fields aredefined as follows:

Reactive Field Region: the region immediately surrounding the antenna where the reactive field energy - the standing wave is dominant.

Radiating-Field (Fresnel) Region: the region between the near-field and the far-field where the radiation fields are dominant, and the field is dependent on the distance.

Far-Field (Fraunhofer) Region: the region farthest away from the antenna, the field distribution is essentially independent propagating waves.

The size of these fields is determined by Antenna Size and the Wavelength of Transmission.

Antenna Propagation

The shape of the propagation from the antenna will be defined by the type ofAntenna Design used. There are basically 3 types of Antenna.

Isotropic: an antenna pattern defined by uniform radiation in all directions

Directional: characterized by more efficient radiation in one direction than another

Omnidirectional: which is uniform in a given plane. E or H (Power or Magnetic)

Antenna Patterns and their Parameters

Below shows a typical radiation pattern, with directional arrays enabling the degrees of radiation and the power levels to be compared. In an ideal design, the radiation would be focused in one direction to give maximum gain. Adding ‘director’ elements will improve & narrow focus and it should also reduce the extra lobes.

The lobes are defined as:

Radiation Lobe: a clear peak in the radiation intensity. (Main Lobe)Minor Lobe: any radiation lobe other than the main lobe.Side Lobe: a radiation lobe in any direction other than the direction(s) of intended radiation.Back Lobe: the radiation lobe opposite to the main lobe.

Antenna Propagation Field Strengths

As can be seen in the above diagram the circles represent the power levels measured in dB with -3dB being maximum gain. Effort is made to reduce the lobes in the side, minor and rear directions.

James Clerk Maxwell (Maxwell Equations for precise measurements and calculations) developed mathematical calculations for the theory of magnetic radiation fields. There are many books on this subject for the dedicated engineer but in most cases, a rule of thumb for ‘Beam Width’ is used: -

‘In a radio antenna, the half power beam width is the angle between the half power -3dB points of the main lobe when referenced to the peak effective radiated power ofthe main lobe.’

The basics show that if you want to transmit or receive a signal which is positioned at230 degrees and you have a beam width of 15 degrees you can aim anywherebetween 222.5 to 237.5 degrees. i.e. your direction plus or minus half the beamwidth. The wider the beam width the further off target you can be. In the case of an omnidirectional antenna, you do not need to aim the antenna, however, the powermay need to be increased to reach your target.

In multi-element antenna the beam width and therefore the gain can be increased byproducing reduced lobes while increasing the main radiating lobe. These antennasare ideal for one point of transmit or receive. In the typical Internet of Things scenario, youare looking to have one central node with multiple point remote nodes. In that case,an omnidirectional antenna is ideal.

To conclude, for most product designers the antenna is a purchased off the shelf withplacement in the design remaining their primary consideration. However, selectingthe best antenna for the application still requires an understating of the polardiagrams supplied with the antenna and those produced by the test house duringcertification. In a live situation, the propagation will be affected by the enclosure of theproduct, the environment including proximity to metallic objects and absorption bynatural and manufactured obstacles.

Siretta understands the challenges faced and have developed their own Antennaselector tool http://www.siretta.com/antenna-selector/ to reduce time to market. Ourportfolio includes cellular modems & terminals, routers, cellular network analysers,RF antennas including solutions for WLAN, LoRa and Sigfox. We offer RF cableassemblies and RF accessories. Frequencies are typically within the 75MHz –5.8GHz range covering the HF, VHF, ISM, GNSS frequencies.

We also offer bespoke customer solutions and our design services are supported byan experienced team of dedicated development & application engineers as well assoftware specialists offering complete end to end solutions with a heavy emphasis onhigh-level system design.

Siretta are a leading developer and manufacturer of Industrial IoT products, software and solutions.
We have extensive knowledge and experience within IoT with a focus on cellular technologies in support of 2G (GPRS), 3G (UMTS), 4G (LTE), NB-IoT and LTE Category M as well as the emerging 5G.